This invention relates to an apparatus for the cleaning of flue gases containing sulfur dioxide that come from circulating fluidized-bed firing systems.
Circulating fluidized-bed firing systems are used in particular for the low-emission combustion of fossil fuels, e.g. coal, peat, wood, and so forth. In burning sulfur-containing coal, for example, the oxidation of the sulfur produces sulfur dioxide, which gets into the atmosphere via the flue gas. These emissions, which are harmful to the earth's atmosphere, are returned to the earth as acid rain by way of the weather cycle. Various procedures have been developed for reducing these harmful emissions to the greatest possible extent.
An overview of the retaining of sulfur dioxide in fluidized-bed firing systems was presented by E. J. Anthony during the “Mediterranean Combustion Symposium” in Antalya, Turkey in June 1999.
For example, a familiar procedure is the addition of a fine-grained alkaline SO2 sorbent, generally limestone (CaCO3), burnt lime (CaO) or also dolomite, into the combustion chamber of the fluidized-bed firing system. Here, first of all the roasting (calcining process) of the limestone to burnt lime (CaO) takes place, and subsequently a reaction occurs between the roasted limestone and the sulfur dioxide of the flue gas.
If in this connection the limestone is exposed to the temperatures of 700° C. to 950° C. that are present in a circulating fluidized-bed firing system, namely if carbon dioxide is driven off from the limestone, then what remains is burnt lime, which because of the driving off of the CO2 has a high degree of porosity and thus a high specific surface area.
The subsequent gas-solid reaction of the burnt lime (sorbent) with sulfur dioxide and oxygen is a surface reaction, and this is why the creation of a high specific surface area is a fundamental prerequisite for this reaction. Remaining behind as a solid reaction product is calcium sulfate or gypsum (CaSO4), which stays in the pores or on the surface of the sorbent or the burnt lime.
Depending on the grain size of the limestone or SO2 sorbent used and on its abrasion properties, either the aggregate of the sorbent-reaction product (lime-gypsum aggregate) remains long enough in the combustion chamber for it to be drawn off via the components of the combustion-chamber ash removal system, or in the case of small particles the sorbent-reaction product aggregate leaves the combustion chamber together with the flue-gas stream and is subsequently separated out in the following flue-gas filter.
The mixture composed of fuel ash, reaction product, and free, unreacted sorbent that is drawn off via the combustion-chamber ash removal system is generally referred to as bottom ash or coarse ash. The particle size of this coarse ash is for the most part larger than 100 μm. The maximum grain diameter can amount to several mm.
The ash carried off with the flue gas that is subsequently separated out in the filter is generally called filter ash. Depending on the quality of the cyclone/separator, the grain size of this ash encompasses the small grain fractions up to about 200 μm in diameter.
From knowledge gained by constructing fluidized-bed firing systems, it is evident that for the degree of desulfurization required in industrial use, namely a reduction in sulfur dioxide of from 70% to 99%, the desulfurization reaction requires a high excess of sorbent. This requirement is all the higher the greater is the demand placed on the degree of desulfurization.
If one uses the Ca/S ratio as a measure for the added limestone or another sorbent, namely the molar quotient of externally supplied Ca and the total sulfur of the fuel that is present, then typical Ca/S values for fluidized-bed firing systems lie between 2 and 4 for a degree of desulfurization of 95%.
This requirement has economic disadvantages, in particular because in general this raises not only the operating costs for the procurement of limestone or of another sorbent, but also the waste-disposal costs for the resulting ash due to the fraction of unreacted sorbent.
In connection with the above-named desulfurization procedure for the flue gas in a circulating fluidized-bed firing system, it has proved to be a shortcoming that the limestone or the SO2 sorbent does not react completely with the sulfur dioxide, since frequently a blanket of gypsum that is almost gas-impermeable forms around the lime aggregate or sorbent aggregate, and also the pores of the lime or sorbent are clogged up by gypsum as the reaction product. The physical/chemical basis for this is the larger molar volume of SO2 diffusing into the lime aggregate or sorbent aggregate compared to the expelled CO2.
Especially in the interior of the grains of the sorbent-reaction product (lime-gypsum grains), a core of unreacted sorbent remains that is no longer available for the reaction, since the reaction partners of sulfur dioxide and oxygen can no longer penetrate down into this core.
The concentrations of unreacted free sorbent in the ash mixture can be as much as 40%, both in the case of filter ash and also with coarse ash, relative to the total ash mixture that is to be carried off. Also, within the framework of the further use of the ash mixture in the cement industry or in roadbuilding, it is desirable to have a lower concentration of free, namely unreacted, sorbent or limestone, to below 3 to 5%.
In current fluidized-bed firing systems, various techniques are being used at present in order to increase the degree of utilization of sorbents or limestone for the purposes of reducing the sulfur dioxide.
Thus, for example, in circulating fluidized-bed firing systems the ash accumulating in the flue-gas filter, which may still contain high proportions of unreacted sorbent, is returned again directly into the combustion chamber.
One drawback of this ash recycling is that the utilization of the still free sorbent in the ash may be of only limited benefit, because the additional dwell time in the fluidized-bed combustion chamber is small and because the reactivity of this ash or of the free sorbent contained in the ash is considerably reduced compared to the original sorbent.
Moreover, a recycling of the bed ash drawn off from the combustion chamber is also customary. To this end, the bed ash is subjected in part to a treatment (sifting of the good grain fraction or grinding up of the bed ash) that is aimed at increasing its degree of reactivity. But this method as well has the drawback that its effect in reducing the requisite consumption of sorbent is very limited, since it does not eliminate the cause of the incomplete reaction, the above-mentioned gypsum blanket around the sorbent or lime aggregate.
By way of the document U.S. Pat. No. 4,312,280, Shearer et al., a system has furthermore been disclosed in which ash from stationary fluidized-bed firing systems is brought into contact with water or steam and is returned to the combustion chamber of the fluidized-bed system. The mixing of the ash with steam and water takes place in a complex fluidized-bed reactor at relatively high temperatures. No reference is made to more extensive process-technology details about operating temperatures of this fluidized-bed reactor or to what water admixtures are used for doing this work. This disclosed system has on the whole a high technological complexity and therefore has not gained much acceptance on the market, partly also because the potential market for stationary fluidized-bed firing systems is limited to small system sizes, and compared to circulating fluidized-bed firing systems they have the disadvantage of having smaller particle dwell times as well as an inhomogeneous temperature distribution.